In order to protect sensitive installations, components and networks against ESD (electrostatic discharge) it is possible to use discrete components with varistor function. These are nonlinear components whose resistance decreases greatly when a specific applied voltage is exceeded. Varistors are therefore suitable for harmlessly dissipating overvoltage pulses. Varistors are produced from a zinc oxide ceramic with grain structure.
It is difficult to integrate varistors in multilayer ceramics and so varistors are usually used as discrete components. Surface-mountable varistors require solderable or bondable metallizations, which then necessarily have to be applied on the zinc oxide surface of the varistor. When producing these contacts, however, the problem arises that solderable and bondable contacts of good quality can be produced only with electrolytic reinforcement. However, the electroplating baths usually used are set up to be acidic or basic and are therefore suitable for dissolving the zinc oxide of the varistors.
An electrolytic coating for producing the solderable or bondable contacts of zinc oxide varistors therefore requires a suitable protective layer for the zinc oxide surface that is not to be coated.
Glass-based protective layers which enable a structured production of contacts have already been proposed. However, it has been found that varistors having a glass coating exhibit a degradation manifested in an intensifying leakage current, also reverse current. Circuits having such varistors exhibit a current consumption increased beyond the permissible value. Furthermore, the degradation may be manifested as a rise in the grain resistance and lead to an increase in the terminal voltage. While the problem of degradation leads only to a relatively slight degradation in the case of thick varistor components, the degradation has an effect all the greater as varistor components become thinner.